This invention relates to an in-situ combustion recovery process within a subterranean, oil-containing formation using high concentrations of oxygen and more particularly to a method for operating a production well in such processes wherein a small amount of an inert gas is continuously injected into the bottom of the well which may be increased to a maximum rate if either the bottomhole temperature of the well or the oxygen content of the effluent gas from the well reach an unsafe level indicating a hazardous condition in the well.
Thermal recovery techniques, in which hydrocarbons are produced from carbonaceous strata such as oil sands, tar sands, oil shales, and the like by the application of heat thereto, are becoming increasingly prevalent in the oil industry. Perhaps the most widely used thermal recovery technique involves in-situ combustion or "fire flooding". In a typical fire flood, a combustion zone is established in a carbonaceous stratum and propagated within the stratum by the injection of air, oxygen-enriched air or pure oxygen through a suitable injection well. As the combustion supporting gas is injected, products of combustion and other heated fluids in the stratum are forced away from the point of injection toward production zones where they are recovered from the stratum and withdrawn to the surface through suitable production wells. U.S. Pat. Nos. 3,240,270-Marx, 4,031,956-Terry, and 4,042,026-Pusch et al are examples of the recovery of oil by in-situ combustion.
In such processes, the prevention of unintended ignition due to the hazardous nature of using pure oxygen is of primary concern. For example, as the combustion zone moves away from the injection well, a large volume of unreacted oxygen sometimes accumulates near the well. If this travels upwardly in the well, a catastrophic fire possibly destroying the well, can be ignited. U.S. Pat. No. 3,125,324-Marx discusses the ignition problem. In addition, U.S. Pat. No. 4,042,026 to Pusch et al disclosed above also discusses the hazardous nature of using pure oxygen in in-situ combustion operations that could lead to uncontrolled reactions or explosions.
U.S. Pat. No. 3,240,270 to Marx discloses an in-situ combustion process for the recovery of oil wherein an inert cooling fluid such as water, nitrogen, or carbon dioxide is injected into the production boreholes so as to maintain the temperature therein below combustion supporting temperature at the oxygen concentration therein and prevent borehole fires.
U.S. Pat. No. 3,135,324 to Marx discloses an in-situ combustion process for recovery of oil wherein a fine dispersion of water is injected with the combustion supporting gas in a sufficient amount to maintain the temperature of the stratum around the injection well below ignition temperature.
It is an object of the present invention to provide a method for safely operating a production well in an in-situ combustion oil recovery operation using high concentrations of oxygen.